Corrosion is defined as the chemical or electrochemical reaction between a material, usually a metal, and its environment that produces a deterioration of the material and its properties. Corrosive attack begins on the surface of the metal. The corrosion process involves two chemical changes. The metal that is attacked or oxidized undergoes an anodic change, with the corrosive agent being reduced and undergoing a cathodic change. The tendency of most metals to corrode creates a major maintenance challenge for metals and metal products, particularly in areas where adverse environmental or weather conditions exist.
Chromium-based anti-corrosive systems containing hexavalent chromium compounds have proven to be an extremely useful and versatile group of chemistries that are extensively used in aircraft metal treatment processes. They impart many beneficial anti-corrosive characteristics to metallic substrates on which they are applied and have been used extensively for the pre-treatment of metals before coating, adhesive bonding and surface finishing. Chemically, chromium-based anti-corrosive systems have involved the combination of hexavalent chromium (e.g., CrO3, CrO4−2, Cr2O7−2) and hydrofluoric acid (HF) in the case of aluminum and its alloys. The hydrofluoric acid removes oxide film from the surface of the metallic substrate (e.g., aluminum) and the hexavalent chromium reacts with the exposed metal and a trivalent chromium oxide precipitates. Using aluminum as an example: Cr2O7−2+2Al0+2H+Cr2O7.H2O+Al2O3.
Chromium oxide, such as that produced according to the above reaction, is quite useful in anti-corrosive applications. It is quite stable in alkaline environments, it is water repellant (hydrophobic) and may act as a barrier coating towards water. Finally, it exhibits a “self-healing effect”—that is, residual hexavalent chromium in the coating may react with damaged areas of the coating—thereby producing more trivalent chromium oxide at damaged sites and therefore “healing” itself. Consequently, chromium-based, and in particular hexavalent chromium-based systems have been extensively used in the aircraft industry because they have proven to be: highly effective at reducing corrosion and as an adhesion promoter for organic coatings and adhesives; particularly resilient as the application/treatment process exhibits a low sensitivity towards variation in process conditions; extremely effective on aluminum alloys; and ensure considerable quality control characteristics as a skilled worker may tell the amount of chromium on the surface of a substrate by mere inspection (color) of the coating.
Concern about chromium—and in particular, hexavalent chromium—in the environment has generated a need to replace chromium-based systems. Therefore “environmentally friendly”, commercially acceptable alternative to chromium-based systems are a welcome addition to corrosion prevention coatings.